Although commercial application of ultrasonic holography has been actively pursued by many persons in the scientific and industrial communities for many years, only limited results have been obtained even though it was once thought that ultrasonic holography held great promise. It was felt that the application of ultrasonic holography was particularly applicable to the fields of non-destructive testing of materials and medical diagnostics of soft tissues that are relatively transparent or translucent to ultrasonic radiation. One of the principal problems that has been encountered and not effectively resolved is the difficulty of obtaining consistent, high quality images. Solutions have been particularly elusive in obtaining undistorted, high quality images of selected focused structures in an object under investigation because of the interference or shadowing of other out-of-focus structures in an object. A typical prior art ultrasonic holographic imaging system for inspecting a selected plane in an object is illustrated in FIG. 1.
Such a typical "real time" ultrasonic holographic imaging system is identified in FIG. 1 with numeral 10. The system 10 is intended to inspect the interior of an object 12. The system 10 generally has a hologram generating sub-system 13 and a hologram viewing sub-system (optical sub-system) 32. Principal components are ultrasonic transducers, generally referred to as an object transducer 14 for generating ultrasonic plane waves 16 to insonify the object 12 and a reference transducer 22 for generating an off-axis beam.
The ultrasonic energy transmitted through the object 12 is directed through a liquid medium to a hologram detection surface 18, which is generally an area of a liquid-gas interface or liquid surface, such as a water surface. Generally the hologram detection surface 18 is physically isolated in a detection container 20 to minimize distortions caused by vibration. The ultrasonic reference transducer 22 generates an off-axis ultrasonic beam that is also directed to the hologram detection surface 18 to form a standing wave hologram. It is frequently desirable to pulse the transducers 14 and 22 at desired intervals to minimize dynamic distortions of the detector surface 18.
Generally an ultrasonic lens assembly 26 is utilized to provide a focused hologram of a desired plane 27 within the object 12. In the example shown, the assembly 26 has a stationary lens 28 having a focal length coincident with the plane of the hologram detection surface 18. A movable complementary lens 30 is provided to be moved to focus on the selected object plane 27 of the object 12 to inspect an internal structures P.sub.0 that is within the selected object plane 27.
The optical subsystem 32 includes a source of coherent light, preferably a laser 34 for generating a beam of coherent light. The laser light beam is directed through a laser lens 36 to achieve a point source that is located at or near the focal point of a collimating lens 38 and then onto the hologram detector surface to illuminate the hologram. The diffracted coherent light radiation containing holographic information is directed back through the optical lens 38 and separated into precisely defined diffracted orders in the focal plane of the collimating lens 38. A filter 42 is used to block all but a first order pattern 44 for "real time" observation by a human eye 46 or an optical recorder, such as a video camera.
FIGS. 2 and 3 illustrate graphically the problem encountered by distortions or image degradation caused by object structures P.sub.1 that are spaced from the focused object plane 27. Such speed object structures are referred to as "out-of-focus" structures, and are identified generally as P.sub.1. The out-of-focus structure P.sub.1, when insonified, retransmits spherical waves and a shadow that is transferred by the optical system 26 to a holographic pattern P.sub.1 in the detection surface as illustrated in FIG. 2. FIG. 3 illustrates the phenomenon in more detail in which the out-of-focus structure P.sub.1 forms a shadow identified as P.sub.10 in the plane 27. The shadow P.sub.10 is in effect imaged on the detector surface of the hologram 18.
One of the principal objects of this invention is to minimize and neutralize those out-of-focus structures of the object such as P.sub.1 to provide a more consistent, high quality image structure in the selected focal plane 27 of the object. A further object is to minimize the distortion caused by such out-of-focus structures utilizing rather inexpensive techniques and apparatus.
These and other objects and advantages of this invention will become apparent upon reading the following detailed description of the preferred and alternate embodiments of this invention.